The present invention relates to the general field of rolling bearings in a turbomachine. More particularly, it relates to feeding oil to pressurized oil films in the rolling bearings of an aviation turbomachine.
An aviation turbomachine has a plurality of rolling bearings (ball bearings or roller bearings) housed in lubrication enclosures and serving to support rotation of the rotary shafts of the turbomachine (e.g. the shafts of the high- and low-pressure spools of a two-spool turbojet). Thus, a two-spool bypass type turbojet generally comprises one rolling bearing housed in a lubrication enclosure situated at the front (referred to as the “front bearing”), and two rolling bearings housed in lubrication enclosures situated at the rear of the turbojet (referred to as “rear bearings”).
In order to obtain satisfactory operation of those rolling bearings, it is necessary firstly to lubricate and cool the rolling elements therein by injecting oil between their rings, and secondly to damp the vibration generated by the rotation of the shafts supported by those bearings by feeding oil under pressure to an oil film that is formed between the outer ring and a structural part of each bearing (referred to as a “squeeze film”).
The oil used for lubricating and cooling the bearings and the oil feeding the vibration-damper films comes from a single oil circuit. Typically, such a circuit comprises an oil duct connecting the oil supply to the various lubrication enclosures that contain the rolling bearings of the turbomachine, an oil feed pump for taking oil from the supply and delivering it to the rolling bearings, and an oil recovery pump for recovering the oil that has been fed to the rolling bearings and returning it to the supply.
The oil feed pump of such a circuit is generally a positive displacement rotary pump that is driven in rotation by an outlet of the accessory gearbox of the turbojet, in turn driven by taking power from the high pressure shaft of the turbomachine. The flow rate of the pump is therefore directly proportional to the speed of rotation of the engine. However, over certain operating ranges of the engine, needs in terms of oil flow rate are not the same for all of the rolling bearings of a turbomachine. In particular, the front bearing of the turbojet, and more particularly its oil layer formed to damp vibration, needs to be fed with oil under pressure at an operating speed that is slower than idling (referred to as the sub-idling speed).
To do this, a known solution consists in dimensioning the oil feed circuit in such a manner that the requirements in terms of pressure and oil flow rate are covered for all operating speeds, in particular at the sub-idling speed. Nevertheless, with such a solution it is necessary to reinforce the feed and recovery pump elements of the oil circuit, thereby increasing its weight. The pressure level required at sub-idling speed implies very high pressures at full throttle. Another known solution consists in dedicating a special pump to the front bearing. In addition to constituting an additional potential source of failure, the impact of such a pump on the weight balance is not negligible.